An inhalation device that ejects fine liquid droplets of medicine into an air path, through which air inhaled through a mouthpiece flows, by using an ejection principle of an ink-jet method, and allows a user to inhale the ejected fine liquid droplets, has been developed (for example, refer to Japanese Patent Application Laid-Open No. 2004-290593 and Japanese Patent Application Laid-Open No. 2004-283245). Such an inhalation device has an advantage capable of spraying precisely a predetermined amount of the medicine in a uniformed particle size.
As basic components of such a medicine ejection device, there are an ejection head in which an ejection energy generation element such as a heater element is disposed, and a medicine tank that contains the medicine supplied to the ejection head.
In the case of preserving an amount of medicine equivalent to multiple inhalation actions in the medicine tank, it is necessary to prevent a concentration change and quality deterioration of the medicine every after use. Therefore, a configuration of atmospheric communication, which is usually used in an ink tank for use in an ink-jet printer, cannot be adopted for the medicine tank, and high gas barrier property and hermetic sealing property are required. The same also applies to the case that a medicine in which contact with air is not desirable is used.
In this case, a flexible container may be used as the medicine tank, and may be formed so as to collapse following to the ejection (Japanese Patent Application Laid-Open No. 2004-290593). For example, aluminum is evaporated onto a polyester film, whereby the medicine tank can be formed. Further, in a case where a body of the medicine tank is a glass container, the medicine tank may be formed in closing one end of the body with a rubber plug so as to be capable of reducing a capacity thereof following the ejection.
In such a configuration, the capacity of the container is increased and decreased when a pressure difference between an inside and outside of the container exceeds a predetermined value. For example, in a case of the glass container, when force applied to the plug owing to a negative pressure inside the tank exceeds the maximum static friction between the glass container and the plug, the plug begins to move in a direction to reduce the capacity of the medicine tank. On the contrary, in the case where a pressure in the tank is a positive pressure, the plug begins to move in a direction to increase the capacity.
Here, in the event of starting to eject the medicine, it is desirable that the pressure in the medicine tank be within an appropriate range. Description thereof is made in detail in the following. As the negative pressure in the medicine tank is increased, that is, as the pressure inside the tank becomes lower than the atmospheric pressure outside the tank, ejection performance from the ejection head is lowered. In the case of ejecting the medicine by means of a head with a nozzle diameter of 3 μm, an ejection amount thereof is not decreased until the negative pressure in the medicine tank becomes approximately −5 kPa. However, it has been understood that the ejection amount is reduced little by little when the negative pressure in the medicine tank exceeds this value, and when the negative pressure reaches −20 kPa, the medicine tank draws in the air from the ejection head, and becomes incapable of ejecting the medicine. On the contrary, in a state where the inner pressure of the tank is the positive pressure, there is a risk that liquid leakage may occur, and hence not desirable. Therefore, it has turned out that, in order to realize stable ejection, it is desirable that the inside of the medicine tank be surely set at the negative pressure from an initial period of the ejection, and that the negative pressure be maintained at a predetermined value or less as much as possible. The predetermined value is −5 kPa in the above-mentioned example.
However, owing to an environmental change while the device is not used, the pressure inside of the medicine tank in the event of starting the ejection sometimes becomes the positive pressure.